An image sensor is a device including several million photoelectric conversion devices, and transforms light into an electric signal depending upon the intensity of the radiation when it receives the light. The image sensor is installed in a digital input device that enables an image to be digitalized to transfer the digital image. Recently, the need for these devices has significantly increased for various security devices and portable digital devices.
The image sensor includes a pixel array in which a plurality of pixels are arranged in a matrix form, and each pixel includes a photo-sensing device and a transmitting and signal output device. The image sensor can be broadly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, depending upon the transmitting and signal output device. The CMOS image sensor concentrates outside light through a microlens, the concentrated light is transmitted to a photo-sensing device such as a photodiode, and the signal is output.
The CMOS image sensor is further refined to accomplish higher resolution, and recently research has focused on decreasing the pixel size to 1 μm or less. It is necessary to consider the geometric optic and wave optic aspects in relation to the microlens due to the small pixel size of 1 μm or less, since the size of the unit pixel reaches up to about 1.5 times the visible ray wavelength. Further, as the unit pixel becomes smaller, the diameter of the microlens becomes smaller. Accordingly, the crosstalk phenomenon with an adjacent pixel more easily occurs unless the focal distance of the lens is decreased, and as a result the resolution is deteriorated. In order to solve this problem, it has been suggested that the thickness of each layer formed between the photodiode and the microlens should be decreased.